The objective of this invention is a heat source that provides heat without the need for a stove, fire, external fuel source, or electrical or other power source. The heat source should be safe to store, transport, and operate; be convenient to use; have minimal weight and volume yet generate sufficient heat for various applications; and be readily and safely disposable after use. A number of portable heat sources, particularly for applications to the heating of food, are known. The materials and methods previously used in such portable heater sources suffer from a number of disadvantages, including the formation of flammable and/or toxic by-products that are potentially dangerous or that may require special disposal as hazardous materials. Many of the previously known portable heat sources also have low efficiency of heat production, i.e., low heat generated for a given weight or volume of heater material.
This invention relates to a heat source, useful in portable heating devices, in which the energy for generation of heat is stored in the form of materials that can be made to react producing heat. More specifically, heat is generated by addition of water to a heat-producing composition. The heat-producing composition of this invention and heaters made therefrom utilize a unique combination of chemical reactants to provide a heat source with the desired properties, but which avoids the disadvantages of prior art heat sources. The present invention provides a great improvement in safety and efficiency of heat production over the prior art.
U.S. Pat. No. 3,079,911 discloses a heating device which generates heat by the oxidation of a metal, and which is activated by addition of a liquid, preferably water. The exothermic composition disclosed is a mixture of aluminum, copper sulfate, potassium chlorate, and calcium sulfate. However, reaction of this mixture leads to the production of gases, which may be flammable or corrosive.
U.S. Pat. No. 4,809,673 discloses the use of the hydration of calcium oxide (quicklime, CaO) to generate heat. The heat output per weight (of the dry material) is approximately 501 Btu/lb. The disadvantages of this type of heater are the relatively low heat output and the requirement for use of a large heater because the powder density of calcium oxide is low.
U.S. Pat. No. 4,753,085 discloses several reactions for use in chemical heaters. For example, the reaction of sodium hydroxide with hydrochloric acid is disclosed, and this reaction produces more heat per weight of heater material (565 Btu/lb) than the hydration of calcium oxide discussed above. However, this heater involves the handling of a strong acid, HCl, which is dangerous. Another reaction disclosed is oxidation of iron powder to produce heat. This reaction is hindered by water. Portable heaters that function well in the presence of water are more desirable because water serves both to transfer heat from the heater to the food or other object to be heated, particularly by evaporation/condensation, and to limit the temperature of the heater by removing the heat of vaporization once the boiling point of water is reached.
U.S. Pat. No. 4,559,921 discloses a self-heating container including a vessel for food. Below the vessel is a sealed container holding calcium oxide and water. The calcium oxide and water are kept separate by a sealed pouch. A tearing element affixed to the pouch opens the pouch and the container, allowing water to contact the calcium oxide thereby starting the exothermic reaction to heat the food.
U.S. Pat. No. 4,949,702 discloses a self-heating device including a heater within a container. The heater includes two parts: a pyrogen of high energy density having a large heating value; and a firing agent which contacts the pyrogen. Both the pyrogen and the firing agent are a mixture of one or more kinds of metal oxides and one or more elementary substances or alloys of metal and semi-metal. The firing agent is activated by a spark from an igniter, something like a match.
U.S. Pat. No. 4,895,135 discloses a self-heating container which generates heat by an exothermic hydration reaction. The container includes an outer shell, an envelope to hold the exothermic reactant, a water bag containing water, and a container body for food. The container body is made from a sheet member, such as metal foil, and a synthetic resin layer attached on at least one side of the metal foil. The sheet member is folded so that its cross-section is W-shaped and it is heat-sealed along the vertical and upper edges. An inverted V-shaped part of the sheet member makes a compartment for holding the envelope inside. The patent refers to the use of hydration of calcium oxide to generate heat.
U.S. Pat. No. 5,355,869 discloses a self-heating assembly for heating group-sized meals, for example meals for a military group. The assembly includes a number of heating trays and a corresponding number of heater assemblies. Each heater assembly is made of a sturdy polymeric sheet of material to form a number of pockets, and a sheet of porous non-woven scrim is attached to the bottom of the polymeric sheet to seal the pockets. A Mg--Fe alloy is the exothermic chemical used in the heater. Water is not included in the assembly but is added when the assembly is ready to be used.
U.S. Pat. No. 5,205,277 (and corresponding European Patent No. 0564680A1) disclose a self-heating container which employs three heating packs. The first heating pack contains calcium oxide and is the main component for producing heat. The second (medium) temperature heating pack contains an exothermic liquid composed of NaCl, acetic acid, and water. The third (high) heating pack contains an exothermic liquid composed of the same components as the medium heating pack, except in different ratios. The liquids in the medium and high heating packs are used to react with the calcium oxide, thereby releasing the heat of hydration to heat food.
U.S. Pat. No. 4,751,119 discloses a container for self-heating or self-cooling drinks or food which includes a device for delivering a liquid reactant to a solid reactant, yielding either an exothermic or an endothermic reaction. The exothermic reactants disclosed are the following: "quicklime, sodium hydroxide, cobalt, chromium, iron, iron hydroxide, magnesium, manganese, molybdenum, tin oxide (II), titanium, sodium, calcium hydroxide, sulfuric acid, nitric acid, metallic sodium, etc. Among them, a powder of magnesium chloride is preferable. The reactants are those generating an oxide reacting with oxygen at room temperature in the form of a revived metal or a metallic [sic] compound and having an exothermic characteristic. It is preferable to mix two or more metal powders." The preferred method of this patent, reaction of magnesium chloride with water, avoids the use of either a strong acid or base. This reaction has a heat output per weight of reactant of only 721 Btu/lb. The complete hydration of magnesium chloride requires a large amount of water, thereby significantly increasing the weight of the heater (if the water is carried with the heater).
U.S. Pat. No. 4,819,612 discloses a container capable of heating its contents when ignited (by a match, for instance). The container holds Japanese sake, coffee, soup or other edible material and in a separate compartment contains a self-combustible exothermic material which may be a mixture of an oxidant and a combustible material. The oxidants disclosed are potassium permanganate, manganese dioxide, trilead tetraoxide, barium peroxide, bromates and chlorates. The combustible compounds disclosed are metal powders of iron, silicon, ferrosilicon, aluminum, magnesium, and copper. The preferred exothermic material is a mixture of potassium permanganate and one or more metals. As noted, the reaction is not activated by water, but by firing a fuse in the container via a match or lighter. The temperature can exceed 1000.degree. C., thereby necessitating precautions to avoid melting the container.
U.S. Pat. No. 4,522,190 discloses a heater material for heating food and other items. It is known as the flameless ration heater (FRH) The heater is a composite of supercorroding metallic alloy powder distributed throughout a porous ultra-high-molecular-weight (UHMW) polyethylene. The supercorroding metallic alloy preferably is a powdered alloy of magnesium and iron, which when wetted with an electrolytic solution, e.g. aqueous sodium chloride, produces heat. This reaction is accompanied by the evolution of flammable and potentially explosive hydrogen gas. This system uses a magnesium/iron alloy, in a matrix of (UHMW) polyethylene producing heat by the following reaction: EQU Mg+2H.sub.2 O.fwdarw.Mg(OH).sub.2 +H.sub.2
The heat output of this reaction is 5,643 Btu/lb (dry weight). Normally, magnesium reacts very slowly with water, because of the presence of an oxide coating on the surface, which prevents further reaction. Iron is added to increase the rate of reaction with water. The major disadvantage of this system is the production of significant quantities of hydrogen gas. The FRH produces 9-10 L of hydrogen gas (at standard temperature and pressure) when used to heat one meal or U.S. Army meal, ready-to-eat (MRE). This volume of gas, which must be vented along with some of the steam produced by the heater, is an inconvenience for the user.
The quantity of hydrogen produced by the FRH device has discouraged its use in the consumer market. For example, a dangerous situation would easily occur if a functioning heater were placed in an operating microwave oven because the amount of hydrogen produced in the confined space of the oven could easily fall within the explosive limits for hydrogen in air.
U.S. Pat. No. 5,117,809 discloses a heater material utilizing the same alloy of magnesium and iron as described in U.S. Pat. No. 4,522,190, but with a different packaging arrangement. Hydrogen gas is nonetheless produced on use of the alloy. The patent also describes the use of other known exothermic reaction materials: calcium oxide, anhydrous calcium chloride, magnesium oxide, zeolite molecular sieves and silica gel--all of which react with water to give off heat.
U.S. Pat. No. 5,220,909 discloses a self-heating container which includes a tub for food. A tray containing an exothermic-chemical pad, composed of a super-corroding Mg--Fe alloy dispersed throughout a porous polymer matrix, and a pouch containing an electrolytic solution which activate the chemical pad, is welded below the tub. A pull-tab is affixed to the pouch so that the electrolytic solution contained in the pouch can be released to initiate the exothermic reaction to heat the food.
The heat source of the present invention differs from the above described systems because it employs a unique combination of chemicals to produce an amount of heat per weight of heater greater than that of many other portable heaters; does not require use of any liquid strong acids or strong bases; and does not produce any flammable by-products. The end products of the exothermic reactions are preferably close to neutral in pH and may be disposed of after use by conventional means, including disposal in a sanitary landfill. The heater of this invention is lower in weight and volume than many previously used systems.